Voltage-gated calcium and sodium currents of starburst amacrine cells in the rabbit retina
The voltage-gated calcium and sodium currents of starburst amacrine cells were examined in slices of the adult rabbit retina. ON-center starburst amacrine cells were targeted for whole-cell recording by prelabeling the retina with the nuclear dye 4′-6-diamidino-2-phenylindole hydrochloride (DAPI). Calcium currents were isolated using an external Ringer that contained tetrodotoxin to block sodium currents and barium to block potassium channels. When starburst amacrine cells were stepped to holding potentials positive to −50 mV, a series of voltage-dependent calcium currents were activated. The calcium current peaked at −10 mV. The calcium currents kinetics were mainly sustained in nature, showing only a small amount of slow inactivation. Nickel (100 [mu]M), a T-type channel blocker, had no effect on the calcium current. Application of the L-type channel agonist BAY K8644 (1–2.5 [mu]M) had small variable effects on the calcium current while the L-type channel antagonist nifedipine (10 [mu]M) had no effect. However, addition of a reported N-type calcium channel antagonist, omega-conotoxin G6A (1 [mu]M), blocked a large portion of the calcium current, as did a more nonselective antagonist, omega-conotoxin M7C (200 nM). Agatoxin 4A (500 nM) reduced a smaller sustained calcium current component, implying a P/Q-type calcium channel was present on these neurons. In addition to the calcium currents, a fast voltage-gated sodium current was observed in many starburst cells. This current could be blocked by tetrodotoxin (200–500 nM). The differing kinetics and durations of the sodium and calcium currents could play important roles in the regulation of synaptic release and in the coordination of spiking by starburst amacrine cell dendrites during retinal development and in the encoding of motion across the retinal surface.(Received November 13 2000)
(Accepted August 13 2001)
Key Words: Starburst amacrine cell; Voltage-dependent calcium channels; Voltage-dependent sodium channels; Direction selectivity; Action potential.
c1 Address correspondence and reprint requests to: Ethan D. Cohen.
p1 Current address: Department of Molecular and Cellular Biology, Box 171, Harvard University, 16 Divinity Avenue, Cambridge MA, 02138, USA. E-mail: email@example.com